1. Field of the Invention
This invention relates to an extra-low iron loss grain oriented silicon steel sheet, and more particularly to an extra-low iron loss grain oriented silicon steel sheet suitable for use in electrical machinery and equipment having excellent heat stability, compressive stress dependence of magnetostriction and lamination factor.
2. Related Art Statement
Lately, remarkable developments and efforts for satisfying the improvement of electrical and magnetic properties in grain oriented silicon steels, particularly ultimate demand on reduction of iron loss are gradually producing good results. However, when using such grain oriented silicon steel sheets, it is a serious problem that the degradation of the above properties is unavoidably caused when the steel sheet is subjected to a so-called strain relief annealing after its working and assembling and the use application is restricted considerably and undesirably.
Throughout the specification, the invention will be described with respect to developmental results on new measures for advantageously satisfying the above demands irrespectively of a high temperature heat treatment such as strain relief annealing, particularly for profitably providing desirable compressive stress dependence of magnetostriction and lamination factor in grain oriented silicon steel sheets.
As is well-known, the grain oriented silicon steel sheet, wherein secondary recrystallized grains are highly aligned in {110}&lt;001&gt; orientation, namely Goss orinetation, is mainly used as a core for transformer and other electrical machinery and equipment. In this case, it is required that the magnetic flux density (represented by B.sub.10 value) is high, the iron loss (represented by W.sub.17/50 value) is low and, in addition to this superior magnetic properties, the magnetostriction property and lamination factor are excellent.
Since these grain oriented silicon steel sheets are usually manufactured through many complicated steps, inventions and improvements are applied to the above steps, whereby low iron loss grain oriented silicon steel sheets having B.sub.10 of not less than 1.90 T and W.sub.17/50 of not more than 1.05 W/kg when the product thickness is 0.30 mm or B.sub.10 or not less than 1.89 T and W.sub.17/50 of not more than 0.90 W/kg when the product thickness is 0.23 mm are manufactured up to the present.
Lately, supreme demands on the reduction of power loss become considerable in view of energy-saving. Particularly, a system of "Loss Evaluation", wherein the reduction percentage of iron is converted into a load on the cost of the transformer in the manufacture of low loss transformer, is widely spread in Europe and America.
Under the above circumstances, there has recently been proposed a method wherein local microstrain is introduced into the surface of the grain oriented silicon steel sheet by irradiating a laser beam onto the steel sheet surface in a direction substantially perpendicular to the rolling direction after the finish annealing to thereby conduct refinement of magnetic domains and hence reduce the iron loss (Japanese Patent Application Publication Nos. 57-2,252, 57-53,419, 58-5,968, 58-26,405, 58-26,406, 58-26,407 and 58-36,051).
Such a magnetic domain refinement is effective for grain oriented silicon steel sheet not subjected to strain relief annealing in the manufacture of stacked lamination-core type transformers. However, in case of wound-core type transformers, the strain relief annealing is performed after the magnetic domain refinement, so that the local microstrain produced by laser irradiation on purpose is released by the annealing treatment to make the width of magnetic domains wide and consequently the laser irradiating effect is lost.
On the other hand, Japanese Patent Application Publication No. 52-24,499 discloses a method of producing an extra-low iron loss grain oriented silicon steel sheet wherein the surface of the grain oriented silicon steel sheet is subjected to a mirror finishing after the final annealing or a metal thin plating is applied to the mirror finished surface or further an insulation coating is baked thereon.
However, the mirror finishing for improving the iron loss does not sufficiently contribute to the reduction of iron loss in comparison with remarkable cost-up of the manufacturing step. Particularly, there is a problem on the adhesion property to the insulation coating indispensably applied and baked after the mirror finishing. Therefore, such a mirror finishing is not yet adopted in the present manufacturing step.
Further, there is proposed a method, wherein the steel sheet surface is subjected to the mirror finishing and then a thin coat of oxide ceramics is deposited thereon, in Japanese Patent Application Publication No. 56-4,150. In this method, however, the ceramic coat is peeled off from the steel sheet surface when subjected to high temperature annealing above 600.degree. C., so that it can not be adopted in the actual manufacturing step.
Moreover, the magnetostriction of the grain oriented silicon steel sheet is a phenomenon that the steel sheet is subjected to stretching vibrations durng the magnetization of the steel sheet, which is a most serious cause of the occurence of noise in the transformer.
The magnetostriction behavior results from the fact that the magnetization process of the steel sheet includes 90.degree. boundary displacement and rotation magnetization. That is, the magnetostriction increases in accordance with compressive stress applied to the steel sheet.
Since the compressive stress is irreversibly applied to the steel sheet in the manufacture of the transformer, it is advantageous that tension is previously applied to the steel sheet in view of the compressive stress dependence of magnetostriction. Of course, the application of the tension to the steel sheet is effective for improving the iron loss in the grain oriented silicon steel sheet, and its effect is conspicuous.
In general, the grain oriented silicon steel sheet is subjected to tension by a double coating consisting of a forsterite layer, which is produced by high temperature reaction between an iron oxide of fayalite (Fe.sub.2 SiO.sub.4) usually formed on the steel sheet surface through decarburization and primary recrystallization annealing before secondary recrystallization and an annealing separator composed mainly of MgO in the final annealing, and an insulation coating produced on the forsterite layer and composed mainly of phosphate and colloidal silica, whereby the magnetostriction property is improved. However, it can not be said that the compressive stress dependence of magnetostriction is sufficiently improved by such a conventional method.
In order to improve the magnetostriction property, there has been attempted the development of insulation coating capable of applying an elastic tension to the steel sheet surface (Japanese Patent Application Publication No. 56-521,117 or 53-28,375). However, such an attempt is still lacking in effectiveness.
Further, the lamination factor of the grain oriented silicon steel sheet is expressed by an amount (percentage) of base metal effectively contributing to magnetic properties, which is obtained by removing forsterite layer and vitreous insulation coating from the surface of the steel sheet during the final annealing. It is said that the increase of such a lamination factor in the grain oriented silicon steel is one of the most important properties. In general, it is known that the surface roughness of the steel sheet is made as small as possible or the thickness of each of the forsterite layer and vitreous insulation coating is made thin for increasing the lamination factor of the product. However, although the thinning of these coatings increases the lamination factor, it is very difficult to stably form a thin coating having a good surface appearance and excellent adhesion property and uniformity at the actual manufacturing step, so that there is a limit in the increase of the lamination factor.